The present invention is directed generally to the formation of a layer on a substrate and specifically to removing surface protrusions from such a layer which inherently occur during its formation.
Processes for depositing a thin film layer on a substrate are known in the art. An example of one such process is physical vapor deposition (PVD). Inherent in the PVD process is the formation of surface protrusions on the thin film during deposition of the material forming the thin film. These surface protrusions can be many times the size of components to be later deposited on the thin film. As a result, the surface protrusions may project into layers of material formed on the thin film layer. In such cases, the surface protrusions may result in an unwanted short circuit between the thin film layer and a layer formed on top of the thin film layer. For example, in the baseplate of a field emission display (FED), such surface protrusions could result in a short circuit between the thin film layer on which emitters are formed and an extraction grid positioned above the emitters. This is true because a surface protrusion can have a height which is much greater than the height of an insulating layer positioned between the thin film layer and the extraction grid.
Various techniques have been attempted in the prior art in an effort to alleviate the adverse effects of surface protrusions. First, the parameters of the PVD process have been adjusted to try and prevent formation of the surface protrusions. This technique has not been entirely successful in that some surface protrusions are inherently formed during the PVD process. Given that some surface protrusions are formed, chemical-mechanical planarization (CMP) has been utilized to try and remove these protrusions from the thin film layer. When CMP is used directly on the thin film layer, however, the larger surface protrusions sometimes break lose and scratch the surface of the thin film layer. These scratches can result in unacceptably large areas of the thin film being unsuitable for the formation of the desired components.
In view of the problems associated with these processes for removing surface protrusions from a thin film, it is desirable to develop a process which removes the surface protrusions from the thin film without detrimentally affecting the surface of the thin film layer.
The present invention is a method for removing a surface protrusion projecting from a layer of a first material deposited on a surface of a substrate. In one embodiment, the method comprises the steps of applying a layer of a second material on the layer of first material. A sufficient quantity of the second material is removed to expose the protrusion. The first material exposed through the protrusion is then removed.
The step of removing a sufficient quantity of the second material to expose the protrusion can comprise mechanical planarization of the second material, chemical mechanical planarization of the second material, or can comprise the steps of removing the second material above a predetermined distance from the surface of the substrate so that the thickness of the second material above the first material is greater adjacent the protrusion than above the protrusion and isotropically removing the second material to expose the protrusion.
In accordance with another embodiment of the present invention, a field emission display (FED) is constructed from a process comprising the following steps. First, a thin film layer is deposited on a substrate. The thin film layer may have one or more surface protrusions. The thin film layer is covered with a sacrificial layer having a top surface. Through a leveling material removal process, such as chemical-mechanical planarization, a portion of the top surface of the sacrificial layer is removed until the sacrificial layer has a predetermined thickness D to thereby expose on the top surface all surface protrusions having a height of at least D. The exposed surface protrusions are then etched to remove the surface protrusions from the thin film layer. The sacrificial layer is etched to remove the sacrificial layer from the thin film layer. Emitters are then constructed on the thin film layer, and an extraction grid is formed above the emitters. Finally, a screen is constructed above the extraction grid, the screen having a phosphor-coated surface facing the extraction grid.